This invention refers to a drill bit for drilling holes in composite materials consisting of fiber/resin laminates.
Among said laminates fiber glass reinforced plastic is included. However the invention is particularly suited for laminates consisting of a resin reinforced with carbon fibers of graphitic structure.
More particularly this invention is directed to laminates hereinafter indicated as "carboresins" formed by graphite fibers embedded in a matrix of epoxy resin.
A material of the type mentioned above is widely used, together with metal sheets, particularly in the aviation industry, for making both movable and fixed parts of airplanes due to its particular characteristics of being light and mechanically resistant.
In practice said carboresin laminates, of a few millimeters in thickness, are used in combination with sheet metal (generally aluminum). This combination is obtained by coupling such laminate and metal sheet face to face and then securing the combination through rivetting.
The combination must guarantee a perfect fluid seal: in fact no type of fluid (liquid or gas) should be capable of penetrating through the coupling of materials so obtained. The rivets, therefore, must be capable of sealing the two adjacent sheets in an absolutely perfect way without there being the possibility of infiltrations of fluids through the seats (i.e. the circular holes in the carboresin sheets) of the rivets themselves.
The drilling of the holes in carboresin laminates presents a great deal of technical difficulties. In fact the graphite fibers/epoxy resin laminate is formed by two phases having very different mechanical properties, i.e., the graphite fibers with a high modulus of elasticity and high mechanical strength and the epoxy resin matrix with low modulus and low mechanical strength.
These differences do not allow working on the composite material using conventional drill bits. In fact, due to the contrast between high elastic modulus and low resistance it is necessary that the stresses on the composite material caused by the penetrating and cutting tool be as small as possible, in order to avoid delamination, particularly of the last lamina at the backside of the sheet.
The bits conventionally used for metal materials, e.g., the bits for wood and the center-point helical bits, cause delamination of the sheet backside due to their geometry, yielding of the resin joining the last lamina to the previous one and a large stress concentration on such lamina.
The best suited bit profile for reducing the force of penetration, according to the conventional prior art, appears to be a bit profile having a high conicity. As a matter of fact the high conicity of the sides of said bits provides the appropriate force of penetration of the cutting edge in the material, with an axial thrust reduced with respect to normal helical bits. However, the front cutting edges often cause delamination along an area around the hole, which is larger than the surface of the hole itself. The delamination on unidirectional composite materials with this type of bit is caused also by the tool sides, because the axial force required in order that said sides may remove material is not sufficiently small. The conicity cannot be further increased, otherwise the bit would become too long, creating problems of dimensions during drilling of the hole.